The present invention relates to an FM receiver having a quadrature FM detecting circuit.
FIG. 1 is a schematic block diagram of a prior art FM receiver having a quadrature FM detecting circuit, in which the excess of an amplitude component of an input FM signal is eliminated by a limiter 1 and then the FM signal is inputted to a quadrature detecting circuit 2. The detecting circuit 2 comprises a phase shifter 21 and a multiplier 22. The phase shifter 21 is arranged to have an amount of phase shift corresponding to the instantaneous frequency of the FM signal. This is accomplished by having the phase shift amount to be 90 degrees with respect to the center frequency of the FM signal (the FM-IF signal frequency of 10.7 MHz). A multiplication of the output of the phase shifter and of the FM signal is performed in the multiplier 22 and the output of the multiplier 22 is amplified in an amplifier 3 so as to form a stereo composite signal.
In order to derive a sub-signal (the difference between the left and right channel signals) from the stero composite signal, there is provided a sub-signal demodulating circuit 4 which comprises a phase-locked loop (PLL) circuit 41 and a multiplier 42. The PLL circuit 41 produces a subcarrier signal of 38 kHz which is synchronized with a stereo pilot signal. The subcarrier signal is multiplied in the multiplier 42 by the stereo composite signal produced by the amplifier 3. The output of the multiplier 42 and the output of the amplifier 3 are applied to a matrix circuit 5 from which left and right channel signals are derived separately from each other.
In such an arrangement, assume now that the FM signal at the output of the limiter 1 is expressed by the following equation for the sake of simplification: EQU e.sub.1 (t)=sin .omega.t (1)
Then the signal e.sub.2 (t) after passed through the phase shifter 21 is expressed as follows: EQU e.sub.2 (t)=A(.omega.) sin {.omega.t+.phi.(.PHI.) } (2)
where A(.omega.) designates the frequency characteristic and .phi.(.omega.) the phase characteristic. Accordingly, the output e.sub.3 (t) of the multiplier 22 is expressed as follows: ##EQU1## In this case, the gain of the multiplier 22 is assumed to be unity.
By causing the signal e.sub.3 (t) expressed by equation (3) to pass through a low pass filter (LPF) to eliminate the second term of equation (3), the first term alone can be derived as a detection output. That is, a signal cos.phi.(.omega.) having an amount of phase shift .phi.(.omega.) corresponding to the instantaneous angular frequency .omega. of the FM signal e.sub.1 (t)=sin.omega.t derived from the quadrature detecting circuit 2. The amount of phase shift is set such that it becomes .phi.(.omega..sub.o)=.pi./2 with respect to the center frequency .omega..sub.o of the FM signal and the detector output at that time becomes cos.phi.(.omega..sub.o)=0. Therefore a detector output can be obtained proportional to .+-..DELTA..omega. with respect to .omega..sub.o .+-..DELTA..omega..
The thus obtained detector output is a so-called stereo composite signal and expressed as follows: EQU e.sub.3 (t)=(L+R)+(L-R) sin.omega..sub.s t+P sin (.omega..sub.s /2)t (4)
where L and R are left and right channel signals respectively, .omega..sub.s is the angular frequency of the subcarrier signal (here equal to 38 kHz), and P the amplitude of the stereo pilot signal. In order to obtain the sub-signal (L-R) from this stereo composite signal e.sub.3 (t) a multiplication on the output sin .omega..sub.s t of the PLL circuit 41 and the e.sub.3 (t) of the multiplier 42 is as follows: ##EQU2## In equation (5), the audio component is (L-R) which means that the sub-signal is demodulated. Thus, the main signal (L+R) in the composite signal and the sub-signal in the equation (5) are subject to addition and subtraction in the matrix circuit 5 so as to produce the left and right channel signals.
Such an arrangement requires demodulating processing in two stages in which a stereo composite signal e.sub.3 (t) is obtained first in the quadrature detecting circuit 2 and then a sub-signal e.sub.4 (t) is demodulated from the composite signal e.sub.3 (t) and a 38 kHz subcarrier signal. As the result, there is a disadvantage that the signal transmission line becomes lengthy so as to deteriorate the S/N ratio and increase the distortion. Further, in multiplying the composite signal and the subcarrier signal, a method is employed in which the composite signal is subject to switching by using a rectangular wave of 38 kHz for the sake of simplification of the circuit. There is another disadvantage in this case that there may occur a beat of the odd order high harmonics of the retangular 38 kHz wave and a signal of a nearby broadcasting station, resulting in beat interference.